Among the known techniques for the (co)polymerization of fluorinated olefinic monomers, optionally in association with non-fluorinated olefins, the most widely used, also on an industrial scale, are those in aqueous emulsion and in suspension, in the presence of radical initiators.
In the case of emulsion (co)polymerization, the polymer is produced in the form of particles dispersed in an aqueous medium by a suitable surfactant. This allows to dissipate the reaction heat very efficiently, hence achieving a good control of reaction temperature and thus a high productivity. Moreover, the absence of organic solvents implies lower process costs and lower environmental impact.
The aqueous emulsion technique shows, however, some drawbacks due to the reaction conditions required. In fact, the use of radical initiators which decompose thermally makes necessary to adopt relatively high reaction temperatures, ranging from at least 50.degree. C. even to 150.degree. C. Polymerization temperatures of this kind negatively influence the characteristics of the final product, in particular they cause a lowering of the second melting temperature and therefore a limitation in the maximum operating temperature of the polymer (the so called "rating temperature").
Such an inconvenience is particularly evident in the case of partially hydrogenated polymers. For instance, it is known that vinylidene fluoride homopolymer shows much more defects of monomeric inversion as far as the polymerization temperature is higher. The increase of such defects leads to a decrease of the crystallinity percentage and thus of the second melting temperature, which, as known, determines the maximum operating temperature of the product. Similarly, for ethylene/tetrafluoroethylene copolymers and especially ethylene/chlorotrifluoroethylene copolymers, an increase in the polymerization temperature implies a drastic decrease in comonomer alternation, with formation of blocks which worsen both mechanical performances and thermal stability of the product. This fact explains why the emulsion (co)polymerization technique is not used for the synthesis of ethylene/chlorotrifluoroethylene copolymers, for which suspension technique at a temperature lower than 25.degree. C. is used instead.
For the time being, the only available technique to lower the polymerization temperature still using the emulsion reaction is generating radicals by redox systems. In the case of fluorinated polymers, however, such technique leads to unsatisfactory results, since it causes formation both of great fractions with low molecular weight and of molecules having polar end-groups, which cause discolouration of the polymer and/or favour dehydrohalogenation, with disastrous consequences on the product quality.
A further drawback of the aqueous emulsion polymerization technique is the need of working at high pressures, generally around 25 bar or even up to 90 bar, with evident drawbacks in plant design. Such high pressures are necessary to increase concentration in the reaction medium of fluorinated monomers, scarcely soluble in the aqueous phase. In such a way it is tried to avoid, as much as possible, the formation of fractions having low molecular weight, which negatively affect mechanical properties of the final product. It is indeed known that, to obtain a good control of molecular weight distribution, it is necessary to reach an optimal balance between concentration of radicals generated by the initiator and concentration of the monomers in the reaction site. Because of the scarce solubility of monomers in the reaction medium, it is therefore necessary to increase the reaction pressure and contemporaneously to carefully dose the initiator, without unacceptably jeopardizing, however, the process productivity.
As regards suspension polymerization of fluorinated olefinic monomers, it allows to employ reaction pressures lower than those necessary for emulsion technique, since monomer solubility in the reaction medium, usually formed by organic solvents such as chlorofluorocarbons, is sufficiently high. The use of organic solvents constitutes, however, a remarkable drawback from a plant viewpoint and implies problems of environmental impact, especially when chlorofluorocarbons are employed.
With respect to emulsion polymerization, by means of suspension technique it is also possible to lower reaction temperature, provided that an initiator active at low temperatures is available. Besides the difficulty of finding for each type of fluorinated polymer such an initiator, in any event it is necessary to adopt particular safety measures, both for synthesis and for shipping and storage, since they are extremely hazardous products, being explosive also at low temperatures. Moreover, such initiators must often be diluted in solvents to avoid an accelerated explosive decomposition.